1. Field of the Invention
The subject invention relates generally to a variable compression ratio engine in which the compression ratio in the combustion chamber of an internal combustion engine is adjusted while the engine is running, and more specifically toward a synchronized, dual crankshaft engine that uses a phase-shifting device to alter the angular position of one crankshaft relative to the other for dynamically varying the engine compression ratio.
2. Related Art
Gasoline engines have a limit on the maximum pressure that can be developed during the compression stroke. When the fuel/air mixture is subjected to pressure and temperature above a certain limit for a given period of time, it autoignites rather than burns. Maximum combustion efficiency occurs at maximum combustion pressures, but in the absence of compression-induced autoignition that can create undesirable noise and also do mechanical damage to the engine. When higher power outputs are desired for any given speed, more fuel and air must be delivered to the engine. To achieve greater fuel/air delivery, the intake manifold pressure is increased by an additional opening of a throttle plate or by the use of turbochargers or superchargers, which also increase the engine inlet pressures. For engines already operating at peak efficiency/maximum pressure, however, the added inlet pressures created by turbochargers or superchargers would over compress the combustion mixtures, thereby resulting in autoignition, often called knock due to the accompanying sound produced. If additional power is desired when the engine is already operating with combustion pressures near the knock limit, the ignition spark timing must be retarded from the point of best efficiency. This ignition timing retard results in a loss of engine operating efficiency and also an increase of combustion heat transferred to the engine. Thus, a dilemma exists: the engine designer must choose one compression ratio for all modes. A high compression ratio will result in optimal fuel efficiency at light load operation, but at high load operation, the ignition spark must be retarded to avoid autoignition. This results in an efficiency reduction at high load, reduced power output, and increased combustion heat transfer to the engine. A lower compression ratio, in turn, results in a loss of engine efficiency during light load operation, which is typically a majority of the operating cycle.
To avoid this undesirable dilemma, the prior art has taught the concept of dynamically reducing an engine compression ratio whenever a turbocharger or supercharger is activated to satisfy temporary needs for massive power increases. Thus, using variable compression ratio technology, the compression ratio of an internal combustion engine can be set at maximum, peak pressures in non-turbo/super charged modes to increase fuel efficiency while the engine is operating under light loads. However, in the occasional instances when high load demands are placed upon the engine, such as during heavy acceleration and hill climbing, the compression ratio can be lowered, on the fly, to accommodate an increase in the inlet pressure caused by activation of a turbocharger or supercharger. In all instances, compression-induced knock is avoided, and maximum engine efficiencies are maintained.
Various attempts to accomplish dynamic variable compression ratios in an internal combustion engine have been proposed. For example, the automobile company SAAB introduced a variable compression ratio engine concept in U.S. Pat. No. 5,329,893. The SAAB concept consisted of a cylinder block and cylinder head assembly connected by a pivot to a separate crankshaft/crankcase assembly, so that a small (e.g., 4°) relative movement was permitted, which movement was controlled by a hydraulic actuator. The SAAB mechanism enabled the distance between the crankshaft center line and the cylinder head to be varied.
Other attempts to accomplish dynamic variable compression ratios have included the operation of synchronized, dual crankshaft engines, wherein the synchronized crankshafts are supported for rotation about parallel axes with their pistons working directly against each other in a common cylinder. Among these so-called “headless” designs which favor opposing pistons working against each other from opposite ends of the same cylinder bore, some are proposed in which the phase relationship of the synchronized crankshafts can be adjusted so that both pistons do not reach top dead center at the same instant. The result is an ability to vary the compression ratio developed by the engine. Examples of synchronized, dual crankshaft engines with phase adjusters may be found in U.S. Pat. No. 6,230,671 to Achterberg, issued May 15, 2001, and U.S. Pat. No. 4,092,957 to Tryhorn issued Jun. 6, 1978, and 4,010,611 to Zachery issued Mar. 8, 1977, and U.S. Pat. No. 2,858,816 to Prentice, issued Nov. 4, 1958.
A particular shortcoming in all prior art attempts to dynamically vary the engine compression ratio by phase-shifting the synchronization of dual crankshafts is the mechanically cumbersome challenge of coupling two crankshafts oriented on polar opposite sides of an engine. Practically speaking, phasing two crank shafts spaced so far apart is very difficult. This leads to complicated and ineffectual mechanisms and designs which are not well suited to today's high efficiency engines and demanding customer expectations. Furthermore, the prior art “headless” designs, in which opposing pistons work against each other from opposite ends of the same cylinder bore, do not readily accommodate the traditional poppet valve nor the time-tested techniques for seating and guiding valves in an internal combustion engine. Thus, gas flow control methods must be employed in such prior art engines at the sacrifice of dependability and economy. And yet again, phase-shifting of dual crankshafts results in a need to vary the timing of gas flow events to conform to “effective” top and bottom dead center timing. The prior art designs significantly complicate any attempts to properly time gas flow events in these complex circumstances. And still further, a primary reason to vary an engine's compression ratio is to take full advantage of turbo- or super-charging systems for high demand conditions. The prior art dual crankshaft engines that enable phase-shifting are notoriously unfriendly to the incorporation of traditional turbo- and super-charging systems that cooperate with the gas flow control system.
Accordingly, there is a need for an improved variable compression ratio engine which enables adjustment of combustion compression ratios on the fly, which is not frustrated by mechanical complexities, and which enables use of more traditional, time-tested valve train and turbo/super-charging techniques.
The two parallel axes crankshafts can be coupled to each other to operate with the same hand, or opposite hands of rotation. Either configuration could be used to achieve the variable compression ratio function, but the configuration that has the crankshafts rotate opposite to each other has the advantage of reduced torsional vibration of the engine assembly. This art is taught in U.S. Pat. No. 2,255,773, to Heftler issued Sep. 16, 1941.